1. Field of the Invention
The present invention relates to a voltage reference circuit.
2. Description of the Related Art
A bandgap voltage reference circuit, which is a sort of a voltage reference circuit widely used in LSIs (Large Scale Integrated circuit), is configured to stably generate a constant voltage independent of the environmental temperature by using the characteristics of the p-n junction. FIG. 1 is a circuit diagram showing an exemplary configuration of a conventional bandgap voltage reference circuit. The configuration shown in FIG. 1 is disclosed in Karel E. Kuijk, “A Precision Reference Voltage Source”, IEEE J. Solid-State Circuits, vol. SC-8, pp. 222-226, June, 1973.
The voltage reference circuit of FIG. 1 includes a PMOS transistor M31, resistors R31 to R33, p-n junction diodes D31 and D32 and an operational amplifier circuit OA3. In FIG. 1, the symbol “VDD” denotes the power supply voltage, and the symbols “VD31” and “VD32” denote the voltages across the diodes D31 and D32, respectively. In addition, the symbols “I31” and “I32” denote the currents through the diodes D31 and D32, respectively. Moreover, the symbol “VIP” denotes the voltage of the non-inverting input of the operational amplifier circuit OA3 (that is, the voltage of the connection node of the resistor elements R31 and R32), and the symbol “VIM” denotes the voltage of the inverting input of the operational amplifier circuit OA3 (that is, the voltage of the connection node of the resistor element R33 and the diode D32). Furthermore, the symbol “VO” denotes the output voltage of the operational amplifier circuit OA3, and the symbol “VOUT” denotes the output voltage of the bandgap voltage reference circuit, that is, the voltage of the connection node of the PMOS transistor M31 and the resistor elements R32 and R33.
In the following, a description is given of conditions for completely eliminating the variation of the output voltage VO depending on the environmental temperature in the voltage reference circuit of FIG. 1. The ratio of the p-n junction area S31 of the diode D31 to the p-n junction area S32 of the diode D32 is defined as:S31:S32=n31:1  (1)and the reverse saturation currents of the diodes D31 and D32 are defined as Is31 and Is32, respectively. Additionally, the thermal voltage is defined as Vt (=kT/q), where the parameter “k” is the Boltzmann constant, where k=1.38×10−23 [m2kg·s2K], the parameter “T” is the absolute temperature [K], and the parameter “q” is the elementary charge, q=1.60×10−19 [C].
Given that the offset voltage of the operational amplifier circuit OA3 is ideally zero for simplicity, the following Equations (2) to (7) are obtained,
                              VO          =                                    VD              ⁢                                                          ⁢              31                        +                                                            (                                                            R                      ⁢                                                                                          ⁢                      31                                        +                                          R                      ⁢                                                                                          ⁢                      32                                                        )                                ·                I                            ⁢                                                          ⁢              31                                      ,                            (        2        )                                                      I            ⁢                                                  ⁢                          31              ·              R                        ⁢                                                  ⁢            31                    =                                    VD              ⁢                                                          ⁢              32                        -                          VD              ⁢                                                          ⁢              31                                      ,                            (        3        )                                                      I            ⁢                                                  ⁢                          31              ·              R                        ⁢                                                  ⁢            32                    =                      I            ⁢                                                  ⁢                          32              ·              R                        ⁢                                                  ⁢            33                          ,                            (        4        )                                                      VD            ⁢                                                  ⁢            31                    =                      Vt            ·                          ln              ⁡                              (                                                      I                    ⁢                                                                                  ⁢                    31                                                        Is                    ⁢                                                                                  ⁢                    31                                                  )                                                    ,                            (        5        )                                                      VD            ⁢                                                  ⁢            32                    =                      Vt            ·                          ln              ⁡                              (                                                      I                    ⁢                                                                                  ⁢                    32                                                        Is                    ⁢                                                                                  ⁢                    32                                                  )                                                    ,                            (        6        )                                          Is          ⁢                                          ⁢          31                =                  n          ⁢                                          ⁢                      31            ·            Is                    ⁢                                          ⁢          32.                                    (        7        )            
By substituting Equations (5) and (6) into Equation (3), the following equation (3′) is obtained:
                                                                                          I                  ⁢                                                                          ⁢                                      31                    ·                    R                                    ⁢                                                                          ⁢                  31                                =                                ⁢                                  Vt                  ·                                      {                                                                  ln                        ⁡                                                  (                                                                                    I                              ⁢                                                                                                                          ⁢                              32                                                                                      Is                              ⁢                                                                                                                          ⁢                              32                                                                                )                                                                    -                                              ln                        ⁡                                                  (                                                                                    I                              ⁢                                                                                                                          ⁢                              31                                                                                      Is                              ⁢                                                                                                                          ⁢                              31                                                                                )                                                                                      }                                                              ,                                                                                          =                                ⁢                Vt                            ⁣                                                ·                                      ln                    ⁡                                          (                                                                                                    I                            ⁢                                                                                                                  ⁢                            32                                                                                Is                            ⁢                                                                                                                  ⁢                            32                                                                          ·                                                                              Is                            ⁢                                                                                                                  ⁢                            31                                                                                I                            ⁢                                                                                                                  ⁢                            31                                                                                              )                                                                      ,                                                                                                        =                                ⁢                                  Vt                  ·                                      ln                    ⁡                                          (                                              n                        ⁢                                                                                                  ⁢                                                  31                          ·                                                                                    R                              ⁢                                                                                                                          ⁢                              32                                                                                      R                              ⁢                                                                                                                          ⁢                              33                                                                                                                          )                                                                                  ,                                                          (                  3          ′                )            It should be noted that the relations of the above-described Equations (4) and (7) are used for obtaining Equation (3′).
By substituting Equation (3′) into Equation (2), the following equation (8) is obtained:
                    VO        =                              VD            ⁢                                                  ⁢            31                    +                                                                      R                  ⁢                                                                          ⁢                  31                                +                                  R                  ⁢                                                                          ⁢                  32                                                            R                ⁢                                                                  ⁢                31                                      ·            Vt            ·                                          ln                ⁡                                  (                                      n                    ⁢                                                                                  ⁢                                          31                      ·                                                                        R                          ⁢                                                                                                          ⁢                          32                                                                          R                          ⁢                                                                                                          ⁢                          33                                                                                                      )                                            .                                                          (        8        )            
When partially differentiating Equation (8) with respect to the absolute temperature T in the both sides, the following Equation (9) is obtained:
                                                                                                              ∂                                          ∂                      T                                                        ⁢                  VO                                =                                ⁢                                                                            ∂                                              ∂                        T                                                              ⁢                    VD                    ⁢                                                                                  ⁢                    31                                    +                                                            ∂                                              ∂                        T                                                              ⁢                                          {                                                                                                                                  R                              ⁢                                                                                                                          ⁢                              31                                                        +                                                          R                              ⁢                                                                                                                          ⁢                              32                                                                                                            R                            ⁢                                                                                                                  ⁢                            31                                                                          ·                        Vt                        ·                                                  ln                          ⁡                                                      (                                                          n                              ⁢                                                                                                                          ⁢                                                              31                                ·                                                                                                      R                                    ⁢                                                                                                                                                  ⁢                                    32                                                                                                        R                                    ⁢                                                                                                                                                  ⁢                                    33                                                                                                                                                        )                                                                                              }                                                                                  ,                                                                          ≅                            ⁢                                                                    -                    2                                    ⁢                                                                          ⁢                  mV                                +                                                                            {                                                                                                                                  R                              ⁢                                                                                                                          ⁢                              31                                                        +                                                          R                              ⁢                                                                                                                          ⁢                              32                                                                                                            R                            ⁢                                                                                                                  ⁢                            31                                                                          ·                                                  ln                          ⁡                                                      (                                                          n                              ⁢                                                                                                                          ⁢                                                              31                                ·                                                                                                      R                                    ⁢                                                                                                                                                  ⁢                                    32                                                                                                        R                                    ⁢                                                                                                                                                  ⁢                                    33                                                                                                                                                        )                                                                                              }                                        ·                    0.08                                    ⁢                                                                          ⁢                                      mV                    .                                                                                                          (        9        )            
Accordingly, the variation of the output voltage VO depending on the environmental temperature is completely eliminated by determining the resistance values of the resistor elements R31 to R33 and the area ratios n31 of the diodes D31 and D32 so that the following Equation (10) is satisfied:
                                                                        -                2                            ⁢                                                          ⁢              mV                        +                                                            {                                                                                                              R                          ⁢                                                                                                          ⁢                          31                                                +                                                  R                          ⁢                                                                                                          ⁢                          32                                                                                            R                        ⁢                                                                                                  ⁢                        31                                                              ·                                          ln                      ⁡                                              (                                                  n                          ⁢                                                                                                          ⁢                                                      31                            ·                                                                                          R                                ⁢                                                                                                                                  ⁢                                32                                                                                            R                                ⁢                                                                                                                                  ⁢                                33                                                                                                                                    )                                                                              }                                ·                0.086                            ⁢                                                          ⁢              mV                                =          0                ,                                  ⁢                                                                              R                  ⁢                                                                          ⁢                  31                                +                                  R                  ⁢                                                                          ⁢                  32                                                            R                ⁢                                                                  ⁢                31                                      ·                          ln              ⁡                              (                                  n                  ⁢                                                                          ⁢                                      31                    ·                                                                  R                        ⁢                                                                                                  ⁢                        32                                                                    R                        ⁢                                                                                                  ⁢                        33                                                                                            )                                              =                                                    2                ⁢                                                                  ⁢                mV                                            0.086                ⁢                                                                  ⁢                mV                                      ≅                          23.25              .                                                          (        10        )            
As is understood from Equation (8), when the resistance values of the resistor elements R31 to R33 and the area ratios n31 of the diodes D31 and D32 are determined so that Equation (10) holds, the output voltage VO is determined to be a constant value that does not vary with respect to the environmental temperature as is given by the following Equation (11):
                                                        VO              =                            ⁢                                                VD                  ⁢                                                                          ⁢                  31                                +                                  23.25                  ·                  Vt                                                                                                        =                            ⁢                                                0.6                  ⁢                                                                          ⁢                  V                                +                                  23.25                  ×                  25.85                  ⁢                                                                          ⁢                  mV                                                                                                        ≅                            ⁢                              1.201                ⁢                                                                  ⁢                                  V                  .                                                                                        (        11        )            It should be noted that the deviation of Equation (11) is based on the fact that the p-n junction forward voltage of the silicon diode is approximately 0.6V, and the thermal voltage Vt is 25.85 mV at the room temperature (27° C.).
In the circuit configuration of FIG. 1, both of the input voltages VIP and VIM of the operational amplifier circuit OA3 are approximately 0.6V. These voltages are lower than the threshold voltage of the generally-available enhancement NMOS transistor (typically, 0.9 to 1.1V). Accordingly, any of the following Measures #1 and #2 is required for surely operating the voltage reference circuit of FIG. 1:
Measure #1: Use depletion-type MOS transistors as the input stage transistors of the operational amplifier circuit OA3.
Measure #2: Raise the power source voltage VDD and use enhancement-type PMOS transistors as the input stage transistors of the operational amplifier circuit OA3. Specifically, the power source voltage VDD fed to the bandgap voltage reference circuit is adjusted to be higher than the sum of the p-n junction forward voltage and the threshold voltage of the PMOS transistor, which is approximately 1.8V.
However, the above-mentioned two measures are not suitable for actual implementations of the bandgap voltage reference circuit. The use of depletion-type MOS transistors (Measure 1) is not desirable in terms of manufacture process simplicity; the use of depletion-type MOS transistors requires a complicated LSI manufacture process, which leads to an increase in the manufacture cost. An LSI integration process is generally designed to manufacture only enhancement-type MOS transistors, since enhancement-type MOS transistors are most-commonly used in LSIs. Although an integration process can be designed to manufacture both of enhancement-type and depletion-type MOS transistors, such integration process requires increased numbers of masks and process steps, thereby causing disadvantages in the cost and TAT (Turn Around Time).
Meanwhile, to raise the power source voltage VDD (Measure #2) is not desirable in terms of reliability and power consumption. In recent years, dimensions of MOS transistors integrated within an LSI is increasingly refined by the advancement of the semiconductor integration technology, and accordingly a voltage allowed to be applied to a MOS transistor is decreased. The raising of the power source voltage VDD is not favorable, since it causes an increase in the voltages applied to MOS transistors. In addition, the raising of the power source voltage VDD results in an increase of the power consumption. A social need for the power-saving of electronic devices is increased in recent years, and for the purpose of satisfying this need, the raising of the power source voltage VDD in an LSI is not desired.
As far as using the circuit configuration of FIG. 1, it is impossible to address the above-mentioned issue by optimizing constants of respective elements of the bandgap voltage reference circuit (for example, a resistance value) or other measures. This fundamentally results from the fact that the voltage fed to the operational amplifier circuit OA3 is excessively low, approximately 0.6V.
Japanese Laid Open Patent Application No. H11-143563 discloses a configuration of a voltage reference circuit that operates on a low power supply voltage. The voltage reference circuit disclosed in this patent document, which is a sort of a bandgap voltage reference circuit, is configured to detect the difference between bandgap voltages across paired diodes in the bandgap voltage reference circuit by using first and second MOS transistor pairs; and to amplify the detected voltage difference. The detected voltage difference is fed back as an electric current to the paired diodes by a pair of a third MOS transistors. This allows an operation on a low power supply voltage, specifically, at a power supply voltage of approximately 1.5V.
The above-described patent document, however, is silent on specific circuit constants and operation conditions for reducing the temperature coefficient substantially to zero.